The present invention relates generally to a cathode ray tube driving amplification circuit in a video signal processing system, and more particularly to a wide-band cathode-ray tube driving circuit for enlarging the driving bandwidth of the cathode-ray tube.
A cathode-ray tube (hereinafter, referred to as a CRT) can be an example of a typical image display apparatus. A CRT driving circuit in the CRT must increase the frequency bandwidth and high-frequency response characteristic of a video signal (or a composite video signal) which is supplied to the tube in order to make possible the improvement to the picture quality displayed thereon. The degree of improvement to picture quality is determined according to both the numbers of scanning lines on a CRT and pixels in each scanning line. Thus, the more the numbers, the more superior the picture quality can be displayed on the CRT.
In the video signal processing system of the standard type CRT having 525 scanning lines, therefore, the driving frequency bandwidth is 4.2 MHz as shown by the formula, that is, the frame scanning frequency (30 Hz).times.the scanning line numbers (525).times.the dot numbers (=266) =4.2 MHz. Accordingly, such standard type CRT driving amplification circuit can display standard video signals on the CRT without any degradation of picture quality if the circuit utilizes a frequency bandwidth of from 4 MHz to 5 MHz.
The conventional CRT driving circuit as shown in FIG. 1 has used a cascode amplifier mainly in a certain high-frequency circuit. The cascode amplifier comprises a common emitter transistor Q2 and a common base transistor Q1 serially coupled to each other.
First, a color difference signal R-Y demodulated and outputted from chrominance signal processor 10 is applied to the base of common emitter transistor Q2 of the driving amplifier 40, and a negative luminance signal (-Y) outputted from luminance signal processor 20 before which both the brightness and contrast are already controlled manually is also applied to the emitter of transistor Q2 via a resistor R5.
The transistor Q2, being of NPN type, inversely amplifies the color difference signal R-Y applied to the base thereof.
Hence, a signal of -(R-Y)+(-Y)=(-R) is high-frequency amplified and generated at a point A which is located at the collector of transistor Q2.
The signal output (-R) is applied to the emitter of common base transistor Q1 and amplified for output at the collector thereof, and the amplified output color signal (R) is applied to cathode image intensity control electrode 31 of CRT 30 via current limiting resistor R2.
In case of the afore-described cascode amplifier, there are difficulties due to amplification factor variance according to an ambient temperature (or direct current gain h.sub.FE), drift due to forward biased junction voltage V.sub.BE and the fluctuation of backward saturation current I.sub.CBO. Furthermore, any change in the amplification factor, the forward biased junction voltage or the backward saturation current varies the output voltage.
Such varied output voltage value is indistinguishable from that which is caused by a change in the input signal voltage, resulting in causing such problems that diminish the stability factor. When a cascode type amplifier is used, however, the video-frequency bandwidth which the CRT driving circuit can cover is approximately 4 to 5 MHz, which is sufficient for use within the general video signal processing system.
Recently, apparatuses that remarkably increase the number of scanning lines or pixels are being developed which display an image of heightened picture quality. For example, there are HD(High Definition)TV, ID(Improved Definition)TV and ED(Enhanced Definition)TV. The afore-mentioned CRT driving circuit utilized in the above exemplified apparatuses require a wide-band CRT driving amplification circuit having a frequency band of approximately from 8 to 20 MHz. Accordingly, when the afore-mentioned cascode type amplifier is used, the problem of not being able to display an image of heightened picture quality arises.